1. Field of the Invention
The present invention is generally in the field of semiconductors. More particularly, the invention is in the field of fabrication of compound semiconductor devices.
2. Background Art
A group III-V semiconductor device, such as a group III-V semiconductor heterojunction field effect transistor (HFET), can utilize a first semiconductor body comprising gallium nitride (GaN) and a second semiconductor body comprising aluminum gallium nitride (AlGaN) to achieve a high current conduction channel between the two semiconductor bodies. The group III-V semiconductor device can include a buffer layer between a silicon substrate and the first semiconductor body, where the buffer layer can comprise a group III-V semiconductor material such as aluminum nitride (AlN). However, forming the buffer layer on the silicon substrate can cause a strain in the first semiconductor body as a result of a difference in crystal lattice structure and thermal coefficient of expansion between the silicon substrate and the buffer layer.
If the first semiconductor body is too thick, the strain in the first semiconductor body can cause cracking in the epitaxial layers in the group III-V semiconductor device and/or cause an undesirable amount of wafer warping during device fabrication. However, for high voltage applications, the first semiconductor body requires a correspondingly high breakdown voltage, which is directly related to the thickness of the semiconductor body. For example, high voltage applications can require a breakdown voltage greater than approximately 300.0 volts. Thus, it is desirable to reduce the strain in the first semiconductor body so that its thickness can be sufficiently increased so as to provide a sufficiently high breakdown voltage for high voltage applications.
In a conventional approach for reducing the strain in the first semiconductor body of a conventional group III-V semiconductor device, a first transition layer comprising AlGaN can be formed over the buffer layer and a second transition layer comprising AlGaN with a lower aluminum composition can be formed over the first transition layer. In the conventional approach, the first and second transition layers can provide some strain reduction in the first semiconductor body. However, additional strain relief in the first semiconductor body may be necessary so as to allow its thickness to be sufficiently increased for high voltage applications without causing epitaxial layer cracking and/or an undesirable amount of wafer warping.